(1) Field of the Invention
The present invention relates to a heavy duty radial tire. In particular, it relates to a heavy duty radial tire having an improved tread pattern, which can prevent irregular wear without deteriorating the wet-resistant property of the tire and is suitable for running the tire for a long period of time.
(2) Description of the Prior Art
A known pneumatic radial tire comprising a tread and a rigid reinforcing structure just under the tread, the reinforcing structure consisting of a carcass which consists of at least one ply having cords arranged therein in the substantial radial plane of the tire, and a belt which consists of at least two cord layers arranged on the outer side of the carcass so as to be crossed with each other at a relatively small angle with respect to the equatorial line of the tire. The tread has at least one zigzag main groove which extends continuously or intermittently on the tread along its circumferential direction, and having continuous or intermittent tread convexes or ribs defined by the zigzag main groove and positioned adjacently to the main groove. However, the pneumatic radial tire causes sometimes irregular wear when the tire is fitted to heavy vehicles, such as truck and bus, and run under a heavy load. The reason is that, as specifically explained in the copending U.S. patent application No. 896,425 by the present inventors, now, U.S. Pat. No. 4,271,885 wear which occurs at the top A of a rib 2 adjacent to main groove 1 in FIG. 1 grows gradually and extends towards the concave corner B of the rib 2, resulting in a stepwise irregular wear along the edge of the rib. This irregular wear is generally called railway wear.
In order to overcome this drawback, the above described U.S. patent has proposed a tread pattern, wherein the amplitude W and pitch P of a zigzag main groove extending on a tread T along its circumferential direction are limited to specifically limited values considerably smaller than commonly used values. Although, this tread pattern is remarkably effective for preventing the above described irregular wear, the pattern deteriorates somewhat tire performance at the same time.
This drawback is specifically the deterioration of traction braking performance under severe use conditions, particularly on wet roads (traction braking performance on wet road is referred to as wet performance hereinafter). This wet performance is no problem under ordinary conditions, but can not be ignored when a heavy duty tire is used under severe conditions.